Music is organized sounds. Sounds have high and low pitches and durations of various lengths. Different pitches sound together will deliver a harmonious effect. That is why music is different form noises of which the latter is unorganized sounds. A composer's work is to organize the sounds to achieve meaningful feelings to humans. Having submerged in music all the time, music composers may subconsciously share melodic patterns in their minds and exhibit similar patterns in their compositions. It is a composer's nightmare: when a composer comes up with a “he-thinks-very-great-chunk-of-sounds (a crude form of melody) and finds out that someone else had already written down and even registered a similar melody.
In reality, two songs may share the identical musical phrases. For example, the first 7 melodic notes of “Sound of Silence” composed by Paul Simon and “Sealed with a Kiss” composed by Gary Geld and performed by Jason Donovan are the same in pitches and durations. Therefore, during the music development process, especially at the final stage before publication, it is often necessary to run a checking process to avoid possible infringement of existing copyrighted musical work.
Similarly, the melodies are not exactly the same but close enough to cause a copyright infringement. To avoid this happening, a checking against already registered melodies is necessary. Checking by ear-listening is not practical.